Physical Modeling of Unsteady Turbulence in Breaking Tidal Bores (original) (raw)
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Unsteady turbulence measurements in breaking tidal bores including the effect of bed roughness
2011
A tidal bore is an unsteady flow motion generated by the rapid water level rise at the river mouth during the early flood tide under appropriate macro-tidal and bathymetric conditions. The present study investigated physically the turbulent properties of breaking tidal bores. The results were based upon some experimental measurements of free-surface fluctuations and turbulent velocities conducted on smooth and rough beds. Using an ensemble-averaging technique, the free-surface fluctuations of breaking tidal bores were characterised. Immediately prior to the roller, the free-surface curved gradually upwards, and the passage of the bore roller was associated with some large water elevation fluctuations. The turbulent velocity measurements were performed at several vertical elevations during the breaking bore. Both the instantaneous and ensemble-averaged velocity data highlighted some strong flow deceleration at all elevations. Close to the bed, the longitudinal velocity component became negative immediately after the roller indicating the existence of a transient recirculation. The vertical velocity data presented some positive, upward motion beneath the roller with increasing maximum vertical velocity at increasing elevation above the bed. The transverse velocity data show some large fluctuations with a non-zero ensemble-average after the roller passage, suggesting some secondary turbulent motion advected behind the bore.
Unsteady Turbulence in Tidal Bores: Effects of Bed Roughness
Journal of Waterway, Port, Coastal, and Ocean Engineering, 2010
A tidal bore is a wave propagating upstream as the tidal flow turns to rising. It forms during spring tide conditions when the flood tide is confined to a narrow funneled channel. To date, theoretical and numerical studies rely upon physical experiments to validate the developments, but the experimental data are limited mostly to visual observations and sometimes free-surface measurements. Herein turbulent velocity measurements were obtained in a large-size laboratory facility with a fine spatial and temporal resolution. The instantaneous velocity measurements showed rapid flow deceleration at all vertical elevations, and large fluctuations of all velocity components were recorded beneath the bore and secondary waves. A comparison between undular ͑nonbreaking͒ and breaking bores suggested some basic differences. In an undular bore, large velocity fluctuations were recorded beneath the first wave crest and the secondary waves showing a long-lasting effect after the bore passage. In a breaking bore, some large turbulent stresses were observed next to the shear zone in a region of high velocity gradients, while some transient flow recirculation was recorded next to the bed. The effects of bed roughness were tested further. The boundary friction contributed to some wave attenuation and dispersion, and the free-surface data showed some agreement with the wave dispersion theory for intermediate gravity waves. The instantaneous velocity data showed however a significant effect of the boundary roughness on the velocity field next to the boundary ͑z / d o Ͻ 0.2͒ for both undular and breaking bores. Overall the findings were consistent with field observations of tidal bores and highlighted the significant impact of undular ͑nonbreaking͒ bores on natural systems.
Turbulence measurements in tidal bore-like positive surges over a rough bed
2013
A sudden rise of the water depth in an open channel flow creates a surge propagating in the channel called a positive surge. Positive surges can be observed as natural phenomena when a spring tide enters a funnel-shaped estuary under appropriate tidal and bathymetric conditions: the process is called a tidal bore. In this study, the free surface profile and the unsteady turbulent motion of positive surges were studied physically in a relatively large facility under controlled conditions based upon a Froude similarity. The experiments were conducted in a 12 m long 0.5 m wide rectangular channel mostly covered by a rough fixed gravel bed. The metrology included a combination of acoustic Doppler velocimeters (ADVs) and acoustic displacement meters (ADMs). The initially steady flow conditions were controlled by the water discharge, and the steady flow properties were investigated thorougly using a Pitot tube and ADV. The positive surge was generated by the fast closure of a downstream gate and the bore propagated upstream against the initially steady flow. Both free-surface and velocity measurements were repeated a number of times to perform an ensemble average. The free surface characteristics were studied for both undular and breaking bores. The passage of the bore was associated with large free surface fluctuations, particularly in the case of breaking bores. The free surface properties were in agreement with earlier findings. The bore shape was closely linked with the Froude number and found to be independent of the distance travelled by the bore. High frequency (200 Hz) instantaneous velocity measurements were conducted 6.13 m upstream of the downstream gate. In the breaking bore, the data showed a transient longitudinal velocity reversal next to the bed beneath and immediately behind the bore front. In the undular bore, a deceleration of the streamwise velocity was observed without velocity reversal of the ensemble averaged velocity, but with transient negative values of the instantaneous longitudinal velocity. For both breaking and undular bores, the vertical velocity data trend followed closely the time derivative of the instantaneous water depth. Large velocity fluctuations were observed during the passage of the bore and in its wake. The turbulent integral length scales of the longitudinal and transverse velocity components were similar in magnitude in steady and unsteady flows, with slightly larger values in the unsteady flow on the upper measurements (z/d o = 0.63). The turbulent integral time scales were larger in the unsteady flow. Further velocity measurements were performed immediately upstream of the gate and provided some insights into the time-variation of the velocity field during the bore generation. The turbulent stresses were larger beneath the bore close to the bed. After the bore front passage, the strongest fluctuations in terms of turbulent stresses were observed on the mid-water column and under the wave crests. Overall this study covered a broad range of unsteady flow conditions which were thoroughly investigated.
This report is published by the School of Civil Engineering at the University of Queensland. Lists of recently-published titles of this series and of other publications are provided at the end of this report. Requests for copies of any of these documents should be addressed to the Civil Engineering Secretary. The interpretation and opinions expressed herein are solely those of the author(s). Considerable care has been taken to ensure accuracy of the material presented. Nevertheless, responsibility for the use of this material rests with the user.
Turbulence measurements in tidal bores: Influence of bed roughness
2011
A positive surge results from a sudden change in flow that increases the depth, and a geophysical application is the tidal bore. Mew experimental investigations were conducted in a rectangular channel and detailed unsteady velocity measurements were performed with high temporal and spatial resolution. The experiments encompassed undular and breaking bores in a horizontal slope and both smooth and rough invert conditions were tested. An undular bore was observed for surge Froude numbers less than 1.5. For Fr < 1.3, the undulations free-surface was smooth. For 1.3 < Fr < 1.45 to 1.5, some breaking was seen at the first wave crest. For Fr > 1.5, a breaking bore was observed. Detailed instantaneous velocity measurements showed a marked effect of the surge front passage. A comparison between undular and weak surge data suggested some basic differences. A systematic comparison was conducted to study the effects of bed roughness. The boundary friction contributed to some wave attenuation and dispersion. The instantaneous velocity data indicated a marked effect of the rough screens on the entire turbulent velocity field.
Turbulent mixing in breaking tidal bores: comparison between field and laboratory data
2013
When a river mouth has a funneled shape with a tidal range exceeding 4 to 6 m, the river may experience a tidal bore, that is a series of waves propagating upstream as the tidal flow turns to rising. Recent studies demonstrated the significant impact of tidal bores on the environmental system and the ecology of the estuarine zone. Herein the unsteady turbulent mixing induced by a breaking tidal bore was documented based upon new physical experiments. Detailed free-surface and velocity measurements were performed with a high-temporal temporal resolution. The physical results were compared systematically with a recent field data set (Mouazé et al. 2010). The propagation of breaking tidal bores was associated with a sharp free-surface discontinuity at the bore front, followed by some transient recirculation next to the bed. A key finding was the close agreement in terms of dimensionless free-surface and velocity data between laboratory and field observations.
Environmental Fluid Mechanics
The tidal bore of the Garonne River (France) was investigated on 29 August, 31 August and 27 October 2015, during which instantaneous velocity measurements were performed continuously at highfrequency (200 Hz). The tidal bore occurrence had a marked effect on the flow field and turbulent Reynolds stress data, indicating large shear stresses, together with large and rapid fluctuations, during the bore passage and the early flood tide. Like many natural process flows, the tidal bore flow motion was dominated by coherent structure activities and turbulent events, with significant impact on the natural systems including in terms of sediment processes. Herein a new turbulent event analysis was developed for the highly-unsteady rapidly-varied tidal bore flow. The analysis was based upon basic concepts, in which turbulent bursting events were defined in terms of the instantaneous relative turbulent flux, and the method was extended to the rapidly-varied, highly-unsteady tidal bore motion. The turbulent event data showed relatively close results for most fluxes during the tidal bores. The event duration showed some tidal trend, with longer turbulent events immediately after the tidal bore passage, occurring simultaneously with major sediment erosion processes. The field data set and analyses suggested that a turbulent event analysis may be applicable to highly-unsteady rapidly-varied flows, providing quantitative details into the turbulent bursts that are responsible for major mixing and sedimentary processes.
Turbulence and turbulent events in tidal bores: field observations
2016
A tidal bore is a compressive wave of tidal origin, propagating upstream as the tidal flow turns to rising when a macro-tidal flood flow enters a funnel shaped river mouth with shallow waters. New field measurements were conducted in the Garonne River tidal bore at Arcins in 2015. Instantaneous velocity measurements were performed continuously at high-frequency prior to, during and after each bore. The bore occurrence had a marked effect on the velocity and turbulent Reynolds stress field, with large and rapid fluctuations, during the bore passage and the early flood tide. A turbulent event analysis was conducted in the highly-unsteady rapidly-varied tidal bore flow. The method detects bursting events by comparing the absolute value of an instantaneous turbulent flux with its standard deviation. This analysis, based upon basic concepts, was extended to the rapidlyvaried, highly-unsteady tidal bore flood flow motion. The turbulent event data analysis showed relatively close results for all studies and all fluxes. A very large majority of turbulent events had a duration less than 0.01 s, with on average 20 turbulent events per second. During all studies, the event duration showed some tidal trend, with longer turbulent events immediately after the tidal bore passage, occurring simultaneously with sediment erosion processes.
2015
This report is published by the School of Civil Engineering at the University of Queensland. Lists of recently-published titles of this series and of other publications are provided at the end of this report. Requests for copies of any of these documents should be addressed to the Civil Engineering Secretary. The interpretation and opinions expressed herein are solely those of the author(s). Considerable care has been taken to ensure accuracy of the material presented. Nevertheless, responsibility for the use of this material rests with the user.
Unsteady turbulence in a tidal bore: field measurements in the Garonne River in October 2013
2014
A tidal bore is an unsteady rapidly-varied free-surface flow generated by the rapid rise in water elevation during the early flood tide, when the tidal range exceeds 4.5 to 6 m and the channel bathymetry amplifies the flood tidal wave. This study describes a detailed field investigation conducted in the Garonne River (France). The tidal bore was undular on 19 October 2013 and the bore front was followed by some well-defined whelps. The instantaneous velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage. Altogether the investigation characterised some unusual turbulence transient in a large river system.